The present invention relates to a method for producing finely structured surfaces, in particular for influencing adhesion, the coatings produced in this way and the use thereof, and a method for producing dies for producing said surfaces.
It is known that finely structured surfaces possess increased adhesive capacity. Such structures and methods for producing them are described for example in EP 1 513 904 B1, WO 01/49776 A2 or WO 2009/053714 A1.
A particular capacity of such surfaces is their dry adhesiveness. Dry adhesiveness means, in the context of this text, the development of adhesive forces between surfaces without adhesion-promoting substances, such as adhesives. These adhesive joints are characterized by the fact that they can be removed again without residues.
Adhesive systems of this kind are often based on structuring of the surface. However, it is necessary to distinguish between systems that require a special counter-structure, e.g. hooks and eyes, and systems that can develop adhesive forces with any surfaces.
Adhesive systems of this kind are for example also known in nature, e.g. in the case of gecko feet, or insects (see also WO 01/49776 A2). It is assumed that in such systems the adhesive forces are based on van der Waals forces. The structuring of the surface leads, on contact, to a very marked increase in contact area, and therefore also in the strength of the adhesive forces that develop on contact.
Such surfaces are generally finely structured. This means that they have depressions or projections with a structuring of less than a millimeter. Advantageously they are microstructured surfaces. This means that the structure of the surface, i.e. the projections and/or depressions, have at least one dimension in the region of at least 100 nm, preferably at least 150 nm, especially preferably at least 200 nm, quite especially preferably at least 250 nm and under 1 mm, preferably under 750 μm, especially preferably under 500 μm, quite especially preferably under 250 μm, especially under 100 μm, in particular under 50 μm, even under 30 μm and quite particularly under 20 μm. Advantageously they have at least two dimensions each independent of one another in this range.
These finely structured surfaces can be produced simply and economically using a die for forming polymers or polymer blends.
The polymer blend must fulfill the following properties/requirements:                1. low viscosity of the polymer blend, to permit rapid, complete and uniform flow into the die;        2. preferably a radiation-curing polymer, for immediate and particularly quick curing. With the planned contact times between substrate and structure master (die), thermal or chemical curing is less preferable, if conceivable at all;        3. high molding fidelity and low shrinkage of the material from the state before to the state after radiation curing, as precise structure duplication (i.e. duplication of the structure of the die) is particularly important for the final properties of the substrate surface structure;        4. hardness and elasticity of the cured polymer material, in order to obtain an optimal dry-adhesive surface structure.        
It is therefore a question of a balance between the viscosity of the polymer blend during application of the polymer blend and the elasticity of the cured polymer blend.
However, it must be borne in mind that low viscosity is mostly achieved with low-molecular polymerizable compounds. However, these have a high density of polymerizable groups, and thus lead to a high degree of crosslinking in the polymer, which produces a high hardness, which as a rule corresponds to high brittleness, so that structures cured in this way can no longer be separated from the die, or during separation break away from the fine structures of the die.
However, lowering the density of polymerizable groups, and thus increasing the molar weight, leads to an increase in viscosity, with the following disadvantages:
A method is known from DE 100 01 135 A1 for producing finely structured surfaces, in which a fine structure is embossed in a radiation-curing thixotropic coating compound with a viscosity above 30 Pas, the embossed surface is detached from the embossing device and is radiation-cured.
A disadvantage is that with these high-viscosity coating compounds, fine structures cannot be filled completely, so that only flat structures are formed by a die.
This can be seen from the structure depicted in FIG. 1 of DE 100 01 135 A1, which with a base diameter of about 1.4 μm only has a height of about 150 nm, and thus an aspect ratio (AR; ratio of the height of the structure to the diameter) of only 0.11.
A method is known from DE 10 2007 021249 A1 for producing structured surfaces, in which a coating compound is cured with radiation, wherein a layer with a thickness of about 1 μm remains uncured on the surface owing to oxygen inhibition, and this is then structured by impressing a die and can be finally cured in a second step.
A disadvantage with this method is that, despite the oxygen inhibition, the uppermost layer is not uncured, but is at least partially cured. This causes an increase in viscosity of the coating compound in this uppermost layer, with all the disadvantages already described above for DE 100 01 135 A1.
The problem to be solved by the invention is to provide a method by which finely structured surfaces with high aspect ratios can be produced simply and economically. In addition, a method is to be provided that makes it possible for the dies for producing these structures to be obtained simply, also in particular for producing hierarchic dies.